Move bytes.Equal, runtime.memequal, and runtime.memequal_varlen
to the bytealg package.

Update #19792

Change-Id: Ic4175e952936016ea0bda6c7c3dbb33afdc8e4ac
Reviewed-on: https://go-review.googlesource.com/98355
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>